Method of extracting sodium from amalgams



' F'iledJune 21, 1954 CONCENTFZATION I'M GRAM -ATOM NA/GKAMATOM(NA HQ.) OFAMALGAM SUPPLlED TO THE EXTRACTION COLUMN p U July 9, .1957 G. PIRLOT 2,798,803

.METHOD, OF EXTRACTING SODIUM FROM AMALGAMS 5 Sheeis-Sheet 1 0,4 4 2.3 45678910 50 40 l l 1 llll llll [AL Rzcssuna m M M. Ha, ABS m EXTRACTION COLUMN MAXIMUM WORKING TEMPERATURE IN C- IN THE EXTRACTION COLUMN July 9, 1957 G. PIRLOT ELECTROLYTIC cms -J 5 Sheets-Sheet 2 AMALGAM OF 0.05 G2. AT. NA/GIZ.AT.(NA*HG) MP. 6C.]

AMA 1. GA M AMALGAM DISTILLATE AMALGAM CONDENSATE CONDENSATE 35 5532 NAIGRATOI HG);

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Ha. A854 AT THE 0F 7m: COLUMN AMALGAM' 0F 0.55 GZAT. NA/OIZAT- (NIHHG) MP? 200C TECHNICALLY Puma soolum United States Patent ()fiFice METHOD OF EXTRACTING SODIUM FROM AMALGAMS Georges Pirlot, Uccle-Brnssels, Belgium,

assiguor to Solvay & Cie., Brussels, Belgium,

The present invention concerns an improvement relating to methods of extracting sodium from amalgams produced by electrolysis of aqueous solutions of sodium salts in mercury cathode cells.

It is known that large amounts of sodium are already produced industrially by electrolysis of fused salts. This process requires a very high consumption of power of the order of 13 to 15 kWh/kg. of Na.

Various processes have already been contemplated for the purpose of replacing electrolysis in the dry-way by the extraction of the alkali metal from amalgams obtained economically by electrolysis of an aqueous solution of a salt of an alkali metal in mercury cathode cells.

The difiiculties inherent in these processes result from the low concentration of alkali metal in the amalgams obtained by electrolysis.

In particular, the extraction of the alkali metal from an amalgam by distillation of the mercury has been recommended. This method of operation may be economical particularly if the production of the alkali metal is judiciously combined with a power generator based on the recovery of the heat of condensation and the sensible heat of the mercury separated by evaporation.

A process forming the subject of American Patent N 0. 1,961,160 is based on the great diiference which exists be tween the boiling temperature of mercury and the temperature of decomposition of the amalgams on the one hand, and the boiling temperature of the alkali metals on the other hand. The process resulting therefrom therefore consists in heating the dilute amalgam to a temperature suflicient to volatilise the mercury and decompose the alkali amalgam in such manner as to leave only pure alkali metal as evaporation residue.

American Patent No. 1,961,135 is based on the hypothesis that the dilute product prepared by electrolysis is composed of an amalgam of higher concentration mixed with free mercury. It is therefore proposed therein to carry out the operation in two stages. In the first stage, a relatively low temperature is used in order to evaporate the free mercury; in the second stage a higher temperature is used which is suflicient to decompose the amalgam and evaporate the mercury liberated.

As far as applicant is aware, these processes have never been utilised industrially.

It is, in particular, incorrect to imagine that the efiluent of the electrolytic cells is a mixture of amalgam and free mercury. In the liquid state, a clearly defined amalgam corresponds to every concentration of sodium.

Applicant has endeavored to separate mercury from sodiumamalgams by the processes mentioned above. By

heating a dilute amalgam it is possible to obtain in the liquid state a more concentrated residuary amalgam, and in the vapour state a dilute amalgam.

Applicant has however observed that by carrying out this evaporation in one or more stills, as indicated in the knpwnprocesses, it was not possible to concentrate the residual amalgam without causing the appearance of solid Patented July 9; 1957 amalgams in the condensers, thus preventing the proper operation of the apparatus.

He has moreover observed that the rectification of the amalgam previously concentrated with a view to the ex: traction of mercury therefrom was impossible because of the appearance of solid amalgams at certain points of the column. By raising the distillation temperature above 700 C., and operating at atmospheric pressure or with a slight vacuum, these obstructions can be avoided, but above 600 C. none of the materials at present utilisable for the construction of the apparatus resists the combined corrosive action of sodium and mercury.

Because of these failures applicant has set himself the task of determining the partial pressures of the Na and Hg in sodium amalgams.

Basing himself on the physical data thus established, he then found the reasons for the previous failures and determined a process of extraction which forms the subject of the present invention.

The fusion diagram of sodium amalgams (Hansen: Der Aufbau der Zweistofiiegierungen, 1936, p. 792) shows that the melting point of certain amalgams. is very high. It reaches a maximum of 355 C. for the amalgam containing 0.33 at. gr. Na/at. gr. (Na-i-Hg). The vapour pressure of this amalgam at a temperature of 355 C. is about mm. Hg of absolute pressure. In a two-stage process, comprising preliminary evaporation of mercury and the extraction of the concentrated amalgam in an ordinary rectifier column, it is therefore necessary that the latter should operate at a pressure such that the boiling temperature of the amalgams of high melting point should be higher than their melting temperature. The still of the column must therefore operate at an absolute pressure higher than 110 mm. Hg. If it is desired to produce a final product composed of practically pure sodium, it is therefore necessary for the still of the column to operate at the boiling temperature of sodium at an absolute pressure higher than 110 mm. Hg, that is to say higher than 7 00 C.

As has already been stated above, no known material resists corrosion under the above-described conditions of operation.

The object of the present-invention is to provide a method of extraction of sodium from amalgams which 1 permits operation at temperatures which the materials at present known can withstand, while avoiding the abovementioned disadvantages of previous processes.

According to the invention, the extraction in two stages of sodium from dilute amalgams obtained by electrolysis in mercury cathode cells comprises, in the first stage, the concentration of the dilute amalgam to a sodium concentration higher than 0.39 at. gr. Na/at. gr. of (Na+Hg), the amalgam thus obtained being exhausted of mercury in the second stage by treatment in an extraction column operating at a maximum temperature comprised between 400 and 600 C., and fed with amalgam, previously concentrated, at the top of the column, while the maximum operating temperature of the column cannot fall below 600 C. for a supply amalgam of which the sodium concentration is 0.39 at. gr./at. gr. (Na-l-Hg), nor below 400 C. for a supply amalgam the concentration of which is higher than or equal to 0.48 at. gr. Na/at. gr. (Na-l-Hg) the lower limit temperature of operation being bound to the concentration of the supply amalgam by a substantially linear relation within the limits 0.39and 0.48 at. gr. Na/at. gr. (Na-l-Hg). The first phase of the process comprising the evaporation of the amalgam from the cells in order tobriri g the concentration to more than 0.39 at. gr. Na/at. gr. (Na-i-Hg) can be effected in one or more simple stills not having a rectification zone. Two stills are advantageously used. The distillate from the first still is recycled to the electrolytic cells after condensation; the liquid amalgam leaving this still is introduced into the second, which works at a lower pressure and/ or a higher temperature. The distillate from this second still can also be recycled to the electrolytic cells. Nevertheless, when the concentration of the amalgam is increased sufiiciently, it is more advantageous to reintroduce it into the first still. The concentrated amalgam containing more than 0.39 at. gr. of Na/atIgr. of (Na-l-Hg) is then introduced into the top of the extraction column. As has already been indicated, the condensation of the vapours leaving these stills may be accompanied by the formation of solid amalgams. The tendency to form encrustations in the condensers is the greater, the higher the Na concentration of the vapours.

According to a particularly advantageous modification for the application of the process forming the subject of the present invention, the evaporation in the first stage is effected in a falling film evaporator. In an apparatus of this type, the electrolytic amalgam or an amalgam already concentrated in a first, ordinary still, is distributed over the wall of a metal tube serving as still. In proportion as it advances, the amalgam is concentrated, emitting vapours having an increasing concentration of sodium. Since the distillation apparatus does not effect any rectification, the vapours are combined and the distillate is the result of their being mixed. The sodium concentration of the distillate is lower than that of distillates obtained in ordinary stills. In the latter, the vapours are in equilibrium with the concentrated amalgam leaving the still. In the film evaporator, the diffusion is practically nil, so that vapours emitted have an intermediate concentration between the sodium concentration of the vapours in equilibrium with the incoming dilute amalgam and that of the vapours in equilibrium with the outgoing concentrated amalgam. Condensations of amalgam in the solid state need practically no longer be feared.

The concentration of the concentrated liquid amalgam leaving the film evaporator is defined by the pressure and by the maximum temperature prevailing in the apparatus. For the reasons already explained, the absolute pressure used is higher than 110 mm. Hg. Without exceeding a maximum temperature of 550 C. it is in particular possible, for operating absolute pressures between 110 and 350 mm. Hg, to obtain an amalgam the concentration of which is between 0.6 and 0.5 at. gr. Na/at. gr. (Na+Hg).. It is obviously possible to obtain amalgams of lower concentration by operating at a lower temperature. It will be observed that the obtaining of amalgams of so high a concentration corresponds to the evaporation of about 90 to 95% of mercury contained in the amalgams from the cells. It is therefore at this stage that the recovery of the heat of evaporation of the mercury must be contemplated.

The second stage of the process consists in treating the amalgam previously concentrated to more than 0.39 at. gr. Na/ at. gr. (Na-I-Hg) for the purpose of separating therefrom mercury-free sodium and a very dilute amalgam. This treatment is eifected in an extraction column fed with amalgam on the first plate. Under these conditions, the boiling amalgam, the sodium concentration of which is the lowest, is the amalgam situated on the top plate. The sodium content of this amalgam being higher than 0.33 at. gr. Na/at. gr. (Na-l-Hg), it is likewise the amalgam with the highest melting point present in the column. In order to avoid any solidification in the column, it is therefore sufficient for the boiling temperature of the supply amalgam to be higher than its melting point. For a determined supply amalgam it is therefore possible to define a limit operating pressure above which the condition set forth above is complied with. This limit pressure is equal to the vapour pressure of the supply amalgam at the melting temperature of the latter. This limit pressure itself defines the maximum limit temperature of operation of the still of the column. It is the temperature at which the sodium has a vapour pressure equal to thelimit pressure.

Figure 1 is a diagram of operation of an extraction column operating in accordance with the present inven-' tion, giving the maximum working temperatures in degrees C. and the working pressures in mm. Hg of absolute pressure, plotted against the concentration of the supply amalgam of the column in at. gr. Na/at. gr. (Na-l-Hg).

Figures 2 and 3 illustrate diagrammatically two different continuous installations for the production of metallic sodium according to the invention.

In the accompanying drawings, the curve AB of the diagram in Figure 1 gives the limit concentration below which the concentration of the supply amalgam must not fall, plotted against the maximum temperature of the column, that is to say the temperature of the still.

Moreover, the operating diagram is limited by the straight lines AX and BY. Materials at present known do not in fact enable the maximum operating temperature of 600 C. to be exceeded in apparatuses in which they are brought into contact simultaneously with mercury and sodium.

In addition, at temperatures lower than 400 C., the column must operate at an absolute pressure lower than 0.4 mm. Hg. These conditions are no longer economical and would require an enormous volume of equipment. taking into account the volume of the vapours liberated at such low pressures. The conditions of operation of the extraction column are therefore dependent on the concentration of the amalgam prepared in the first stage of the process. It is seen that the concentration of this amalgam cannot fall below 0.39 at. gr. Na/ at. gr. (Na-l-I-Ig). The amalgam entering at this concentration must be treated in an extraction column the maximum operating temperature of which must be at least 600 C. When the concentration of the incoming amalgam is progressively higher, the limit operating temperature of the column can be progressively lowered until it is possible to attain 400 C. for an amalgam entering with the concentration of 0.48 at. gr./at. gr. (Na-I-I-Ig). Between these two concentrations the limit operating temperature is dependent on the concentration of the incoming amalgam in accordance with the curve AB of the diagram in Figure 1. This curve may as a first approximation be replaced by a straight line. This obviously refers to a minimum limit temperature, and in practice it will be advantageous to keep below this limit by selecting an operating point falling in the surface XABY of the diagram. When the concentration of the amalgam entering is higher than 0.48 at. gr. Na/ at. gr. (Na-i-Hg), the limit operating temperature can be selected at any value between 400 and 600 C.

In the selection of the operating conditions of the extraction column, consideration is paid particularly to the economic necessity of recovering the calorific energy spent and to the sodium concentration of the vapours leaving the apparatuses. The mercury separated from the amalgams must in fact be recycled to the electrolytic cells. As a whole, the vapours condensed must contain a minimum sodium content, although amalgams containing 0.1% of Na by weight can still be returned to the cells. The sodium concentration of the distillates depends on the sodium concentration of the amalgam prepared in the first stage of the process, and also on the conditions of operation of the column. The higher the sodium concentration of the amalgam supplied to the column, the richer in sodium the vapours in equilibrium with this amalgam will be. The sodium concentration of the vapours can be reduced in a film evaporator, but it nevertheless depends on the concentration in sodium of the outgoing amalgam, because the vapours in equilibrium with this amalgam are mixed with the vapours emitted in the top part of the evaporator.

In the extraction column, the vapours emitted have a sodium concentration which directly depends on the concentration of the amalgam prepared in the first stage, because they are in equilibrium with this amalgam on the top plate of the column. From this point of view, it would be advantageous to concentrate the amalgam to the minimum compatible with the resistance of the materials to corrosion. A preliminary concentration of the amalgam between 0.45 and 0.65 at. gr. Na/at. gr. (Na-l-Hg) appears to be favourable.

The extraction of the sodium in the second stage of the process can obviously be effected in two or more extraction columns, but for the reasons indicated above this solution does not appear to be particularly advantageous.

The examples given below show, with the aid of Fig ures 2 and 3 of the accompanying drawings, the characteristics of operation of the installations for distillation according to the process forming the subject of the present invention. These examples are not limitative and only show the possibilities offered for the distillation of amalgams.

Example I Referring to Figure 2, which shows the diagram of a continuous installation for the unitary production of 100 kg. of metallic sodium, 16,730 kg. of amalgam of 0.05 at. gr. Na/at. gr. (Na+Hg) are taken from the cells 1 for the electrolysis of sodium chloride. The melting temperature of this amalgam is 13 C. This amalgam is continuously introduced through the pipeline 2 into a still do constituted by a simple boiler still operating, without reflux, at the temperature of 450 C. and at an absolute pressure of 290 mm. Hg. The vapours leaving this distillation apparatus are condensed in the condenser 4, and the distillate comprising 16,630 kg. of amalgam of 3.5 10- at. gr. Na/at. gr. (Na+Hg), melting at 38 C., is recycled to the electrolytic cells through the pipeline 5. At the botom ot the distillation apparatus 1,503 kg. of. amalgam of 0.41 at. gr. Na/at. gr. (Na-j-Hg) are continuously collected. This amalgam, having a melting point of 316 C., is introduced into the bottom of a second distillation apparatus 3b working at a temperature of 450 C. at an absolute pressure of 12.3 mm. Hg. The vapours leaving this still, which is of a similar design to the first, are cooled and condensed at 6. The condensate represents 900 kg. of amalgam of 0.046 at. gr. Na/ at. gr. (Naa-Hg), melting at C. It is recycled to the foot of the first distillation apparatus. From the distillation apparatus 312, 603 kg. of liquid amalgam of 0.65 at. gr. Na/at. gr. (Na|-Hg), having a melting point of 112 C., are collected and introduced into the top of an extraction column 7 provided with filling elements and working at an absolute pressure of 4 mm. Hg, corresponding to a temperature at the bottom of the column of 500 C.

The vapours condensed at 8, namely 503 kg, are composed of amalgam of 0.10 at. gr. Na/at. gr.

posed of technically pure sodium, of which 100 kg. are collected per unit of time.

Example II Referring to Figure 3, which shows the diagram of an installation for the continuous production of 100 kg. of metallic sodium by the two-stage process, in which there is made use of a film evaporator for the preliminary concentration of the amalgam, 16,800 kg. of amalgam of 0.05 at. gr. Na/at. gr. (Na-l-Hg) are taken per unit of time from the cells 1 for the electrolysis of sodium chloride. This amalgam, having a melting point of 13 C., is continuously introduced through the pipeline 2 into the film evaporator 3c and uniformly distributed over the inside wall of the apparatus, which operates at an absolute pressure of 200 mm. Hg. and is heated to 550 C.

The mixture of vapours produced on the wallof the evaporator is condensed in the condenser 4. 15,965 kg. of distillate, composed of an amalgam of a sodium concentration of 1.3 10* at. gr. Na/at. gr. (Na-i-Hg), are thus obtained. The melting point of this amalgam is -3S C. It is directly recycled to the electrolytic cells through the pipeline 5. At the bottom of the film evaporator, 835 kg. of an amalgam of a sodium concentration of 0.55 at. gr./ at. gr. (Na-j-Hg), having a melting point of 200 C., are continuously collected and are introduced into the top of the extraction column 6 operating at the maximum temperature of 550 C. and at an absolute pressure of 10.8 mm. Hg. The vapours escaping at the top of the extraction column are condensed at 7, thus giving 735 kg. of dilute amalgam of 0.01 at. gr. Na/at. gr. (Na-l-Hg), with a melting point of 42 C., which are mixed with the distillate of the film evaporator for recycling to the electrolytic cells. At the bottom of the extraction column kg. of technically pure sodiumare continuously withdrawn per unit of time.

I claim:

1. A process for the recovery of sodium from a dilute sodium-mercury amalgam by distillation in two stages, which comprises distilling said amalgam in a first stage to raise the sodium concentration of the amalgam to more than 0.39 gr. at. Na/gr. at. (Naj-Hg), and subjecting the concentrated amalgam thus obtained to treatment in a distillation zone operating at a pressure of 0.4 to 30 mm. Hg absolute, the amalgam concentrated in the first stage being supplied at the top of said distillation zone, the maximum temperature in said zone lying within the range 400 and 600 C. with the temperature being at 400 C. when the amalgam supplied has a sodium concentration which is at least 0.48 gr. at. Na/gr. at. (Na-j-Hg), and saidrnaximum temperature being at 600 C. when the amalgam supplied has a sodium concentration of 0.39 gr. at. Na/gr. at. (Na-t-Hg), the temperature varying with the sodium concentration of the amalgam supplied as a substantially linear function between the concentrations 0.39 and 0.48 gr. at. Na/gr. at. (Na-f-Hg).

2. A process for the recovery by distillation in two stages of sodium from a dilute sodium-mercury amalgam produced by the electrolysis of an aqueous solution of a sodium salt in a mercury cathode cell which comprises distilling said amalgam in a first stage to raise the sodium concentration of the amalgam to more than 0.39 gr. at. Na/gr. at. (Na+Hg), and subjecting the concentrated amalgam thus obtained to treatment in a distillation zone operating at a pressure of 0.4 to 30 mm. Hg absolute, said amalgam concentrated in the first stage being supplied at the top of said distillation zone, the maximum temperature in said zone being between 400 and 600 C. with the temperature being at 400 C. when the amalgam supplied has a sodium concentration which is at least 0.48 gr. at. Na/gr. at. (Na+I-Ig), and said maximum temperature being at 600 C. when the amalgam supplied has a sodium concentration of 0.39 gr. at. Na/gr. at. (Na-i-Hg), the temperature varying with the sodium concentration of the amalgam supplied as a substantially linear function line between the concentrations 0.39 and 0.48 gr. at. Na/gr. at. (Na-l-Hg).

3. A process for the recovery of sodium from a dilute sodium-mercury amalgam by distillation in two stages, which comprises distilling said amalgam in a first stage in at least one distillation area without refilux to raise the sodium concentration of the amalgam to more than 0.39 gr. at. Na/gr. at. (Na-i-Hg), and subjecting the concentrated amalgam thus obtained to treatment in a distillation zone operating at a pressure of 0.4 to 30 mm. Hg absolute, said amalgam concentrated in the first stage being supplied at the top of said distillation zone, the maximum temperature in said zone being between 400 and 600 C. with the temperature being at 400 C. when the amalgam supplied has a sodium concentration which is at least 0.48 gr. at. Na/gr. at. (Na-j-Hg), and said maximum temperature being at 600 C. when the amalgam supplied has a sodium concentration of 0.39 gr. at. Na/ gr. at. (Na-f-Hg), the temperature varying with the sodium concentration of the amalgam supplied as a substantially linear function line between the concentrations 0.39 and 0.48 gr. at. Na/gr. at. (Na-l-Hg).

4. A process for the recovery of sodium from a dilute sodium-mercury amalgam by distillation in two stages, which comprises treating said amalgam in a first stage in a film evaporating zone at a pressure greater than 110 mm. Hg to raise the sodium concentration of the amalgam to more than 0.39 gr. at. Na/ gr. at. (Na-l-Hg), and sub jecting the concentrated amalgam thus obtained to treatment in a distillation zone operating at a pressure of 0.4 to 30 mm. Hg absolute, said amalgam concentrated in the first stage being supplied at the top or" said distillation zone, the maximum temperature in said zone being between 400 and 600 C. with the temperature being at 400 C. when the amalgam supplied has a sodium concentration which is at least 0.48 gr. at. Na/gr. at. (Na-{-Hg), and

said maximum temperature being at 600 C. when the amalgam supplied has a sodium concentration of 0.39 gr. at. Na/gr. at. (Na-t-I-Ig), the temperature varying with the sodium concentration of the amalgam supplied as a substantially linear function line between the concentrations 0.39 and 0.48 gr. at. Na/gr. at. (Na-i-Hg).

5. A process for the recovery by distillation in two stages of sodium from a dilute sodium-mercury amalgam produced by the electrolysis of an aqueous solution of a sodium salt in a mercury cathode cell which comprises distilling said amalgam in a first stage in two distillation areas in series to raise the sodium concentration of the amalgam to more than 0.39 gr. at. Na/gr. at (Na-l-Hg), and subjecting the concentrated amalgam thus obtained to treatment in a distillation zone operating at a pressure of 0.4 to 30 mm. Hg absolute, said amalgam concentrated in the first stage being supplied at the top of said distillation zone, the maximum temperature in said zone being between 400 and 600 C. with the temperature being at 400 C. when the amalgam supplied has a sodium concentration which is at least 0.48 gr. at. Na/ gr. at. (Na-l-Hg), and said maximum temperature being at 600 C. when the amalgam supplied has a sodium concentration of 039 gr. at. Na/ gr. at. (Na-l-Hg), the temperature varying with the sodium concentration of the amalgam supplied as a substantially linear function line between the concentrations 0.39 and 0.48 gr. at. Na/ gr. at. (Na-l-Hg), the distillate from the first of said distillation areas being returned to said cell and the distillate from the second of said distillation areas being mixed with the distillate from the distillation zone of the second stage and introduced into the bottom of the first distillation area.

6. A process for the recovery by distillation in two stages of sodium from a dilute sodium-mercury amalgam produced by the electrolysis of an aqueous solution of a sodium salt in a mercury cathode cell which comprises distilling said amalgam in a first stage in a film evaporat ing zone to raise the sodium concentration of the amalgam to more than 0.39 gr. at. Na/gr. at. (Na+Hg), and subjecting the concentrated amalgam thus obtained to treatment in a distillation zone operating at a pressure of 0.4 to 30 mm. Hg absolute, said amalgam concentrated in the first stage being supplied at the top of said distillation zone, the maximum temperature in said zone being between 400 and 600 C. with the temperature being at 400 C. when the amalgam supplied has a sodium concentration which is at least 0.48 gr. at. Na/ gr. at. (Na-H-Ig), and said maximum temperature being at 600 C. when the amalgam supplied has a sodium concentration of 0.39 gr. at. Na/ gr. at. (Na-kHg), the temperature varying with the sodium concentration of the amalgam supplied as a substantially linear function line between the concentration 0.39 and 0.48 gr. at. Na/gr. at. (Na-i-Hg), the distillate from said film evaporating zone being mixed with the distillate leaving said distillation zone and being returned to said cell.

References Cited in the file of this patent UNITED STATES PATENTS 1,961,135 Crahan et al. June 5, 1934 

1. A PROCESS FOR THE RECOVERY OF SODIUM FROM A DILUTE SODIUM-MERCURY AMALGAM BY DISTILLATION IN TWO STAGES WHICH COMPRISES DISTILLING SAID AMALGAM IN A FIRST STAGE TO RAISE THE SODIUM CONCENTRATION OF THE AMALGAM TO MORE THAN 0.39 GR.AT.(NA+HG), AND SUBJECTING THE CONCENTRATED AMALGAM THUS OBTAINED TO TREATMENT IN A DISTILLATION ZONE OPERATING AT A PRESSURE OF 0.4 TO 30 MM. HG ABSOLUTE, THE AMALGAM CONCENTRATED IN THE FIRST STAGE BEING SUPPLIED AT THE TOP OF SAID DISTILLATION ZONE, THE MAXIMUM TEMPERATURE IN SAID ZONE LYING WITHIN THE RANGE 400 AND 600* C. WITH THE TEMPERATURE BEING AT 400* C. WHEN THE AMALGAM SUPPLIED HAS A SODIUM CONCENTRATION WHICH IS AT LEAST 0.48 GR.AT. NA/GR. (NA+HG), AND SAID MAXIMUM TEMPERATURE BEING AT 600* C. WHEN THE AMALGAM SUPPLIED HAS A SODIUM CONCENTRATION OF 0.39 GR. AT. (A/GR.AT.(NA+HA), THE TEMPERATURE VARYING WITH THE SODIUM CONCENTRATION OF THE AMALGAM SUPPLIED AS A SUBSTANTIALLY LINEAR FUNCTION BETWEEN THE CONCENTRATIONS 0.39 AND 0.48 GR. AT NA/GR. (NA+HG). 